On 1/11/23 8:27?PM, Ming Lei wrote: > Hi Stefan and Jens, > > Thanks for the help. > > BTW, the issue is observed when I write ublk-nbd: > > https://github.com/ming1/ubdsrv/commits/nbd > > and it isn't completed yet(multiple send sqe chains not serialized > yet), the issue is triggered when writing big chunk data to ublk-nbd. Gotcha > On Wed, Jan 11, 2023 at 05:32:00PM +0100, Stefan Metzmacher wrote: >> Hi Ming, >> >>> Per my understanding, a short send on SOCK_STREAM should terminate the >>> remainder of the SQE chain built by IOSQE_IO_LINK. >>> >>> But from my observation, this point isn't true when using io_sendmsg or >>> io_sendmsg_zc on TCP socket, and the other remainder of the chain still >>> can be completed after one short send is found. MSG_WAITALL is off. >> >> This is due to legacy reasons, you need pass MSG_WAITALL explicitly >> in order to a retry or an error on a short write... >> It should work for send, sendmsg, sendmsg_zc, recv and recvmsg. > > Turns out there is another application bug in which recv sqe may cut in the > send sqe chain. > > After the issue is fixed, if MSG_WAITALL is set, short send can't be > observed any more. But if MSG_WAITALL isn't set, short send can be > observed and the send io chain still won't be terminated. Right, if MSG_WAITALL is set, then the whole thing will be written. If we get a short send, it's retried appropriately. Unless an error occurs, it should send the whole thing. > So if MSG_WAITALL is set, will io_uring be responsible for retry in case > of short send, and application needn't to take care of it? Correct. I did add a note about that in the liburing man pages after your email earlier: https://git.kernel.dk/cgit/liburing/commit/?id=8d056db7c0e58f45f7c474a6627f83270bb8f00e since that wasn't documented as far as I can tell. -- Jens Axboe
